Differential amplifier circuit and method for reducing thermally induced offsets caused by large differential signals

Abstract

An amplifier includes a differential amplifier (10) having an input stage (20) for amplifying a differential input signal (Vin), and an output stage (6) coupled to the input stage (20) for producing and output signal (Vout). The input stage (20) includes main input circuitry (20A) for amplifying small-signal values of the input signal (Vin) and alternative input circuitry (20B) for amplifying the input signal (Vin) during conditions which cause thermal imbalance in the main input circuitry (20B). The input stage (20) includes switching circuitry (12) for coupling the input signal (Vin) to the main input circuitry (20A) during normal small-signal operating conditions and to the alternative input circuitry (20A) during large-signal operating conditions that cause thermal imbalance in the main input circuitry (20B).

Description

BACKGROUND OF THE INVENTION [0001] The present invention relates generally to problems associated with differential thermal effects caused by steady-state or transient power imbalances in integrated circuit chips, and particularly to circuits and techniques for reducing the time required for recovery from such differential thermal effects. [0002] It is well known that transient or steady-state large-signal input conditions can cause circuit operation that creates large power imbalances within an integrated circuit. In some cases, the integrated circuit must be operated so as to allow it sufficient time to “recover” from the large power imbalances before proper circuit operation can continue. The power imbalances can cause a temporary temperature imbalance that substantially changes the operating characteristics of circuit components such as transistors located in the region of the temperature imbalance. This may result in unacceptably inaccurate circuit performance until enough “rec...

AI Technical Summary

Benefits of technology

[0017] It is an object of the invention to provide a circuit and technique for reducing or preventing transient or temporary power imbalances in an integrated circuit amplifier which cause transient inaccuracies in an output signal produced by the amplifier.

[0018] It is an object of the invention to provide a circuit and technique for reducing or preventing a transient or temporary power imbalance in a particular region of an integrated circuit amplifier which cause a thermal tail of a signal produced by the amplifier.

[0019] It is an object of the invention to provide a circuit and technique for reducing or preventing a transient or temporary power imbalance in a particular region of an integrated circuit amplifier due to an excessively large input signal.

Problems solved by technology

It is well known that transient or steady-state large-signal input conditions can cause circuit operation that creates large power imbalances within an integrated circuit.

The power imbalances can cause a temporary temperature imbalance that substantially changes the operating characteristics of circuit components such as transistors located in the region of the temperature imbalance.

This may result in unacceptably inaccurate circuit performance until enough “recovery time” has elapsed to allow the temperature to be balanced within the region.

Various other conditions are known to cause similar imbalanced thermal conditions in integrated circuit chips.

For example, large-signal slewing conditions or transient imbalances may cause recovery settling “tails” in the output response of a circuit.

Input signal overdrive conditions beyond the linear range of an amplifier and its feedback loop may result in thermal imbalance conditions of long duration.

Also, various special-purpose applications that cause linear feedback in an amplifier circuit to be interrupted or limited (e.g. in sample / hold amplifiers, peak detector circuits, and limiting / clamping amplifiers), can cause thermal imbalance conditions of the kind referred to above in.

Although a thermal imbalance condition can occur due to simple transient slewing, it may be most problematic in applications in which a feedback loop is interrupted for a relatively long interval, for example in a sample / hold amplifier, a peak detector circuit, a limiting or clamping amplifier, or a multiplexed amplifier.

However, if the differential input voltage Vin between the bases of transistors Q1 and Q2 is excessively large, then one of transistors Q1 and Q2 carries much more current than the other, and therefore dissipates much more than the normal amount of power and, therefore increases the temperature of that transistor.

For example, if Vin+ is much larger than Vin−, then input transistor Q1 is turned off and dissipates no power, and a large current flows through input transistor Q2, causing it to dissipate a large amount of power.

However, the thermal recovery time or “tail” shown in waveform “A” of FIG. 6 may be unacceptably long.

This causes a thermal imbalance between cascode transistors Q5 and Q6.

There appears to be no previously known practical solution to reducing the above described long thermal recovery “tails”.

There also is an unmet need for a circuit and technique for electrically detecting signal conditions which would cause transient or temporary thermal imbalances in an integrated circuit without relying on system timing or control signals.

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